It's possible to do something dangeraus and still know what you're doing.
Look at those crazy stunt people like jackass. It's unsafe, but they know what they are doing so they (usually) don't get hurt.
As for the accurcy, I sort of realized that afterwards, when I did more research, but this method will still give you at least an idea of wattage. If you can afford the fancy stuff that does it right good for you, but especially here in Canada it's not obvious to get the fancy stuff, sometimes it's too expensive, sometimes you can't even get it period.